1. Field
The present invention relates to a thin film piezoelectric resonator, a method of manufacturing the same, and a filter including the same.
2. Description of the Related Art
Recently, as wireless technologies have been rapidly developed, a method of achieving high-speed transmission of information has been developed. A radio frequency (RF) has been used together with an increase in an amount of transmitted information, and a radio frequency communication apparatus has been required to have a small size and light weight. Generally, a wireless apparatus includes a RF front end unit that processes a radio frequency, and a base band (BB) unit that processes a digital signal. The BB unit performs modulation and demodulation of a signal by a digital signal process. Basically, since the BB unit may be formed by a LSI chip, the BB unit can be easily small-sized. The RF front end unit performs amplifying or frequency conversion on a radio frequency signal as an analog signal. Since it is difficult for the RF front end unit to be constructed by using only an LSI chip, the RF front end unit has a complicated structure which includes a plurality of passive components, such as an oscillator or a filter.
Generally, as RF (radio frequency) and IF (intermediate frequency) filters in a mobile communication apparatus, a surface acoustic wave (SAW) element is generally used. However, a resonance frequency of the SAW element is inversely proportional to the distance between interdigital electrodes, and in a frequency region exceeding a frequency of 1 GHz, the distance between the interdigital electrodes is 1 μm or less. With respect to a high frequency for a used frequency required in recent times, it is difficult to correspond to the used frequency. Since the filter uses a special substrate containing LiTaO3, it is basically an individual component, and it is difficult to reduce a scale of the filter.
Instead of the SAW element, as a resonator that has been attracted attention in recent times, there is a thin film piezoelectric resonator that uses a longitudinal vibration of a piezoelectric thin film in a thickness direction. The thin film piezoelectric resonator is called a bulk acoustic wave (BAW) element. In the thin film piezoelectric resonator, a resonance frequency is determined by the velocity of sound and the thickness of a piezoelectric film. Generally, the resonance frequency corresponds to 2 GHz at a thickness of 1 to 2 μm, and corresponds to 5 GHz at a thickness of 0.4 to 0.8 μm. That is, the resonance frequency may rise to several tens of giga hertz. Further, the thin film piezoelectric resonator can be easily formed on a Si substrate, and has an advantage in the requirement of downsizing.
In order to operate the thin film piezoelectric resonator, it is preferable for the thin film piezoelectric resonator to have a structure in which a peripheral portion of a piezoelectric thin film is fixed and a central portion thereof freely vibrates. For this reason, a method of forming a thin film piezoelectric resonator having a following structure has been suggested (for example, JP-A 8-148968 (KOKAI)). According to this method, in the thin film piezoelectric resonator, a lower electrode, a piezoelectric layer, and an upper electrode are sequentially deposited on a silicon substrate, an upper surface of the piezoelectric layer contacts the air through the upper electrode, and a lower surface of the piezoelectric layer contacts the air through the lower electrode, and a cavity is provided below the lower electrode, or a cavity for exposing the lower electrode is provided at the substrate.
Further, in order to obtain a sound resonator which is robust and has a large Q value, a method of manufacturing a thin film piezoelectric resonator has been suggested in which a depression is formed in a substrate by etching, a sacrificial material is filled into the depression, a conductive layer and an electrode layer are deposited, and the sacrificial material is removed from the depression (for example, JP-A 2002-140075 (KOKAI)).
For example, as disclosed JP-A 8-148968 (KOKAI), when forming the cavity passing through the substrate, it is required for the substrate to be etched from a rear surface of the substrate. The etching of the substrate from the rear surface is performed by means of wet etching using a drug solution and dry etching using fluorine based gas. In the wet etching, an alkali liquid, such as potassium hydroxide (KOH) to be an anisotropic etching liquid of silicon, or tetramethylammonium hydroxide (TMAH), is used. In the dry etching, a fluorine based gas, such as C4F8, CF4, or the like, is mainly used. When the cavity is formed by means of the above-described etching, if considering a processing margin or stability at the time of mass production, it is required to provide below the lower electrode a stopper layer (hereinafter, referred to as dielectric layer) that can sufficiently ensure Si, such as SiO2 or Si3N4, and etching selectivity. However, considering etching selectivity in etching the rear surface of the silicon substrate, if the dielectric layer is formed to have a large thickness, when the dielectric layer finally remains below the lower electrode, the slight reduction of the film thickness occurs due to the etching from the rear surface of the substrate, and the variation occurs in the resonator characteristic. Further, when the dielectric layer is removed, since the film thickness of the dielectric layer is large, an etching time necessary for each etching is increased, which results in lowering the throughput.
Meanwhile, in a case in which thickness of the dielectric layer is small, a possibility becomes high in which the dielectric layer is also etched due to etching performed when forming a lower electrode, a piezoelectric layer, and an upper electrode on the surface side of the substrate, in particular, when forming the piezoelectric layer, and thus the substrate is exposed to the outside. When the substrate is exposed, mutual diffusion with metals, such as the upper electrode, a bonding pad, and the like, or a silicide reaction occurs, which causes an increase in resistance.